Investigating dielectric failure of A transformer during works testing resulting from inadvertent drying process deficiency

Transformer insulation being hygroscopic always retains some amount of moisture. However, for the reliable service life, the moisture level has to be contained within the permissible level of ≤0.5%. In this context, proper drying of insulation of the active part of the transformer during manufacturing plays a vital role. Any deficiency in processing affects long term reliability of the product. And any major lapses in processing could even lead to test bed failure.

This paper discusses a typical case of test bed failure of a large capacity transformer due to improper processing and inadequate oil impregnation of an oven-dried insulation before subjecting it to high voltage testing at manufacturer's works. The authors share their experience for detecting such deficiencies and analyzing the root cause of such failure through a live case study.

The paper presents a step-by-step approach that was adopted during the failure diagnosis and analysis exercise. While the in-depth investigation process involved various aspects of root cause analysis, like studying adequacy of insulation design, quality of materials, & adherence to the company's process norms throughout the manufacturing activities, etc, the scope of the paper is confined to discussions on the prime cause of failure of transformer.